SAMPEX

Subsequent missions have observed parts of the belts-including SAMPEX, which observed the belts from below-but what causes such dynamic variation in the belts has remained something of a mystery.

SAMPEX measurements showed that, in the first days of November, 2003, the radiation belts-- and the magnetic field lines that confine them--were pushed inward to only half their usual distance, 6,000 miles from the equator.

The SAMPEX results show that the cosmic-ray nuclei are located within the inner Van Allen belt at an altitude of roughly 6,000 kilometers at the equator and that they increase in number as the Sun's activity drops during its 11-year cycle.

The radiation belt identified by SAMPEX joins two others that were discovered in 1958 by physicist James A.

June: The Solar Anomalous Magnetospheric Particle Explorer (SAMPEX), one of NASA's fleet of small-scale science missions, will carry four detectors to probe the energy and ionization state of streams of charged particles moving in Earth's magnetic field.